1. Technical Field
The present invention relates to a virtual image display apparatus which presents an image formed by an image display element and the like to an observer, and in particular to a virtual image display apparatus suitable for a head mount display to be mounted on the head of the observer.
2. Related Art
As an optical system which is incorporated in a virtual image display apparatus such as a head mount display (hereinafter, referred to as an HMD) to be mounted in the head of an observer, various optical systems have been proposed (refer to JP-A-2000-66106, JP-A-2000-131614, JP-A-2000-199853 and JP-A-2004-341411).
With regard to the virtual image display apparatus such as the HMD, it is desirable to have a wide angle of view of an image light and to reduce the weight of the apparatus. In particular, thinning the thickness in a visual axis direction of the observer and making a central point close to the observer are very important in order to improve a wearing sensation.
In addition, in a state in which the observer's view is totally covered and only the image light is viewed, the observer cannot judge the status of the external world, therefore the observer feels uneasy. Then, new various uses of a virtual reality are made by superimposing and displaying the external world with the image. Accordingly, a display in which the image light is superimposed and displayed without obstructing the view of the external world is desirable.
Further, in order to improve the wearing sensation of the observer and enhance the form of an appearance, generally, it is desirable to make an image display device close to glasses and dispose the image display device alongside a face, without placing the image display device in an upper part of the eyes.
In order to make the optical system compact, and separate the image display device from the position of the observer's eyes without obstructing the observer's view, a relay optical system which makes a display image light into an intermediate image in the middle of the optical system, and enlarges and displays the intermediate image may be used.
In JP-A-2000-66106, JP-A-2000-131614, JP-A-2000-199853 and JP-A-2004-341411, by using two prisms having an asymmetric curved surface (a free curved surface) as a relay optical system which corrects the eccentric aberration to form the intermediate image, it is possible to obtain a display optical system of a small and compact configuration.
However, this type of optical system in the related art is not designed such that the external light is intended to be viewed by the observers, and thus the prism in front of eyes is small, so a sufficient external light is not received. Further, since the shape of the curved surface which transmits the external light is not suitable, the diopter scale with respect to the external world does not become zero and thus blurring and distortion occur.
On the other hand, in the optical system in JP-A-2008-158203, surfaces which transmit the external light are parallel flat surfaces, so that a clear view with almost no distortion is achieved. However, the optical system in JP-A-2008-158203 uses a hologram in a half mirror surface, so that an aberration or wavelength selectivity due to this gives an effect to an image light, and thereby lowering the image quality.